The return of the wolf is one of the unexpected ecological
bonuses of the modern era. So numerous are wolves that this fall
Wisconsin and Wyoming have joined Idaho and Montana in opening
wolf-hunting seasons for the first time in years. Minnesota follows
suit next month; Michigan may do so next year. The reintroduced wolves
of Yellowstone National Park have expanded to meet the expanding
packs of Canada and northern Montana.

The same is happening in Europe. Wolf populations are rising in
Spain, Italy and Eastern Europe, while in recent years wolves have
recolonized France, Germany, Sweden and Norway, and have even been
seen in Belgium and the Netherlands. Nor are wolves the only "apex
predators" to boom in this way. In the U.S., bears and mountain
lions are spreading, to joggers' dismay. Coyotes are reappearing
even within cities like Chicago and Denver.

The effect of top predators on lesser predators, like foxes,
raccoons and skunks, not to mention domestic cats, can be
devastating. Wolves may kill deer and cows, but they also kill
these smaller "mesopredators"-middle-of-the-food-chain carnivores.
That may be good news for other creatures, especially birds. The
very presence of large predators can intimidate the mesopredators:
In the Bahamas, large groupers cause small ones to spend more time
in hiding, allowing smaller reef fish to thrive.

In 1988 ecologists coined the term "mesopredator release" for the theory that the
original disappearance of apex predators at the hand of human
beings had caused a population boom in small opportunistic
predators and omnivores. In Africa, for instance, baboons have
boomed where leopards have been exterminated, to the detriment of
antelopes as well as crops. In one marine case, overharvesting of
Atlantic sharks caused an expansion in the number of rays, which in
turn hurt the stocks of scallops.

Now, as exemplified by the wolf, top predators are returning
little by little. This is due to legal protection and the
increasing retreat of people to cities and suburbs (teenagers who
play computer games would once have staked out wolf kills to
protect the family's herd).

So is the return of top predators now suppressing rather than
releasing mesopredators?

In parts of Europe, introduced American mink have harmed birds,
water voles and other waterside wildlife. But now newly abundant
predators of mink, once devastated by DDT, have caused mink
populations to fall. In Finland sea eagles are hunting mink; in
Britain otters are.

Complicating the picture, some species can be either apex
predators or mesopredators. In Yellowstone National Park, coyotes
are mesopredators that appear to have declined at the paws of
wolves, which is good news for rodents and other creatures. But in
suburbs the coyote is more like an apex predator, whose return lays
waste the domestic cats that kill so many birds. Even in rural
areas, the coyote is an efficient predator of foxes, skunks and
badgers. So the arrival of coyotes in an area may be bad for
rabbits but good for birds.

Likewise, raccoons are usually a classic mesopredator, but
controlling their numbers in Florida to save turtle eggs from their
depredations proved counterproductive, because egg-eating crabs
then thrived.

Ecology is a complicated and unpredictable business. To test
whether the revival of large predators is generally good news for
ecosystems, Dr. Laura Prugh of the University of Alaska at
Fairbanks is setting out to compare coyote, fox and lynx
populations in an area with intensive wolf control, compared with
nearby Denali National Park and Preserve, where wolf populations
are intact. As she and her co-writers said in a recent paper, given what programs to
control mesopredators cost, letting apex predators thrive may
provide an "ecosystem service" by controlling them cheaply and more
effectively.